Non-air gases (i.e. gases that are not derived from air) are commonly used in the manufacture of products such as semiconductors, LCDs, LEDs and solar cells. For example, nitrogen trifluoride is used as a chamber cleaning gas, while silane and ammonia can be used for deposition of silicon and silicon nitride respectively during chemical vapor deposition (CVD) processes.
Semiconductor, LCD, LED and solar cell manufacturers often require a supply of non-air gas in the vapor phase, at high or ultra-high purity at a high flow rate with the capability of supplying the gas in the vapor phase in a discontinuous flow pattern. The presence of low-volatility contaminants in these gases (i.e. contaminants that are less volatile than the non-air gas) is particularly undesirable, since they can deposit on the product substrate and deteriorate, or otherwise adversely affect product performance. For example, water, is a common low volatility ammonia contaminant that can deposit on LED sapphire substrates, resulting in reduced LED brightness and yield loss. For such applications, vapor phase moisture levels in ammonia that exceed 1 ppb can be detrimental to the processes, and the products produced thereby.
New semiconductor products have large throughput and consequently require large quantities of non-air gases. Additionally, due to the batch nature of semiconductor process tool operation, the use pattern of non-air gases is often preferably discontinuous.
Many non-air gases are transported and stored as liquids or vapor/liquid mixtures. Such gases are known as low vapor pressure gases and include, for example, ammonia, hydrogen chloride, carbon dioxide and dichlorosilane. Low vapor pressure gases typically have a vapor pressure less than about 1500 psig at a temperature of about 70° F. According to known methods, because low vapor pressure gases are supplied as liquids or vapor/liquid mixtures, a device for heating/boiling these gases is required so that vapor phase product can be supplied to the desired end use, such as, for example, the semiconductor, LED, LCD or solar cell manufacturing process. This boiling is commonly achieved by applying heat to the supply vessel outer wall, as described, for example, in U.S. Pat. Nos. 6,025,576 or 6,614,412. In such systems, vapor phase low vapor pressure gas is withdrawn from the supply vessel. Sufficient heat is applied to boil liquid phase low vapor pressure gas at the rate that vapor phase low vapor pressure gas is withdrawn from the supply vessel, thereby theoretically maintaining supply vessel pressure.
U.S. Pat. No. 6,025,576 describes a configuration whereby vapor phase, low vapor pressure gas is withdrawn from a heated transport vessel that uses heaters that are only in tensioned, non-permanent contact with transport vessel. The contaminants that have a lower volatility than the low vapor pressure gas preferentially remain in the liquid, producing low contaminant level vapor. Vapor is drawn from the vessel until liquefied gas occupies only about 10% volume of the cylinder, which brings the contact area of the liquefied gas to below the heater level.
U.S. Pat. No. 6,614,009 discloses a system configuration whereby vapor phase, low vapor pressure gas is withdrawn from a large heated transport vessel (e.g. isotainer) that includes permanently positioned heaters. These heaters are preferably located so as to minimize direct heating above the lowest expected liquid level to maximize purity. However, the '009 patent does not describe a means to maximize low vapor pressure gas utilization by maintaining a supply vessel in service until the moisture level exceeds some value.
U.S. Pat. No. 6,581,412 describes a system whereby vapor phase, low vapor pressure gas is withdrawn from a heated transport vessel that employs heaters which are in contact with the transport vessel. This patent describes a method for controlling the temperature of a liquefied compressed gas in a supply vessel comprising: positioning a temperature measuring means onto the wall of the compressed gas supply vessel, monitoring the temperature of the supply vessel and controlling heater means to heat the liquefied gas in the supply vessel. However, the '412 patent does not describe a means to identify the appropriate time to remove a supply vessel from service.
U.S. Pat. No. 6,363,728 describes a means for controlling heat input to a low vapor pressure gas contained in a heated transport vessel. The system comprises a heat exchanger disposed on a delivery vessel to provide or remove energy from a liquefied gas, pressure controller for monitoring pressure and a means for adjusting the energy delivered to the vessel contents. However, the '728 patent does not describe a means to identify the appropriate time to remove a supply vessel from service.
A typical, known means of addressing present operational challenges in the industry is to remove the supply vessel from service when the mass of low vapor pressure gas remaining in the supply vessel falls to a pre-set value (typically from about 10% to about 20% of the initial mass). However, this approach fails to recognize that the key liquid level (that is, the liquid level at which a vessel should be removed from service) will be different depending on the key parameter that is selected (vessel pressure, wall temperature or water level).
A significant problem exists in the field, as no useful means exists for determining efficiently when a low vapor pressure gas supply vessel should be removed from service. Presently known systems risk removing a supply vessel from service too early or too late. As a result, if the supply vessel is removed from service too early, low vapor pressure gas will be wasted. If the supply vessel is removed from service too late, several deleterious effects can occur. For example, the contaminant level can build beyond tolerable limits, resulting in adverse effects in the end use, such as, for example, semiconductor, LED, LCD or solar cell manufacturing processes. Such potential adverse effects include, for example, yield loss.